Occupant safety has been a major concern of vehicle manufacturers since the late 1960's due to consumer demand and government regulation. Safety devices required under government regulations on all new automobiles sold in the United States include seat belts, dual front air bags, a collapsible or crushable steering column, head restraints, and side door guard beams. In addition, a representative model of each vehicle sold must be crash tested to demonstrate that the occupants of the vehicle would survive a 30 mile per hour crash into a barrier, which is equivalent to crashing into a parked car at 60 mph.
When worn, seat belts typically provide a small amount of slack for comfort, and to allow for some movement by the vehicle occupant. Therefore, seat belts are generally equipped with a mechanical or pyrotechnic pretensioning device that takes up the slack in the event of a collision. In addition, the webbing material in a seat belt is designed to stretch under a load, such as that which is experienced in a collision. This stretching, combined with any slack not taken up by a pretensioning device, allows the occupant to move forward in a collision before being restrained by the belt. As a result, portions of the occupant's body can impact portions of the vehicle interior, which, correspondingly, may themselves be moving back towards the occupant due to the crushing forces imposed by the collision.
Typical seat belts are relatively narrow, and, as a result, cannot distribute the load resulting from a collision over a wide area of the occupant's body. As a result, the high load that results from the collision is imposed on only a small portion of the occupant's body, which can cause injuries. Moreover, such belts do not provide any control over the motion of the occupant's head and neck, which may be subjected to severe loads.
Air bags are intended to supplement seat belts, and to overcome the deficiencies of such belts. However, because air bags are stored in the dash or steering wheel, the bag must inflate rapidly to deploy in time to restrain the occupant, or an important portion of the space and time available to decelerate the occupant and prevent injury is lost.
Inflatable seat belts have been proposed as a means of overcoming the deficiencies of standard seat belts and air bags. In particular, inflatable seat belts are in contact with the occupant, and, thus, begin to restrain the occupant more quickly than air bags, pushing the occupant back into the seat, and decreasing the total load on the occupant during the collision. However, prior art inflatable seat belts have not been totally successful. Prior art inflatable seat belts often include a pair of inflatable sections, which are difficult to store without interfering with the entry into and exit out of the vehicle by an occupant, and which require large inflators, complicating the placement of the inflator in the vehicle. This difficulty in positioning the inflator requires the inflation gas to be passed through conduits in the vehicle, slowing the inflation time, increasing the amount of gas needed, and, thus, increasing the size of the inflator required.
While the relatively slow inflation rate of prior art inflators is acceptable for the deployment of a dash or steering wheel mounted air bag, inflatable seat belts must deploy in less than one fourth the time available for the air bag due to their proximity to the occupant's body, and, e.g., to provide protection in a side impact, as the vehicle occupant is far closer to the impact than in a frontal or rear-end collision. The proximity of the occupant to the interior of the vehicle in a side impact substantially reduces the time available for deployment of the inflatable belt.
The inflators used with prior art inflatable seat belts are typically pyrotechnic inflators of the type used in conventional steering wheel and dash mounted air bags, which rely solely on a pyrotechnic gas generating composition to produce the inflation gas. Typical prior art pyrotechnic inflators are relatively inefficient thermodynamically, as a large amount of the heat generated by the combustion of the pyrotechnic gas generating composition is transferred to the components of the inflator, rather than to the inflation gas, reducing the final volume and/or pressure of the inflation gas. As a result, such inflators require an excessive amount of pyrotechnic material, as well as insulation between the inflator and the inflatable fabric to keep from burning or weakening the fabric, resulting in an increase in inflator size and additional difficulties in packaging the belt and inflator.
Hybrid inflators, which produce an inflation gas from the combination of the release of a stored pressurized gas and the generation of gas from the combustion of a pyrotechnic gas generation composition, have been proposed as an alternative to pyrotechnic inflators. However, the thermodynamic efficiency of prior art hybrid inflators is also relatively low because a significant portion of the heat generated by the combustion of the pyrotechnic material is absorbed by components in the inflator, rather than by the inflation gas, resulting in size and packaging problems similar to those found in pyrotechnic inflators.
A variety of hybrid inflators are disclosed in the prior art for use with passive restraint systems. For example, U.S. Pat. No. 3,837,671 to Hamilton discloses a compact inflating means for inflating inflatable vehicle restraint systems, such as inflatable bands, i.e., seat belts. The inflating means is a hybrid inflator that comprises a source of pressurized gas and means to release the pressurized gas in response to an increase in pressure in the pressurized gas. The increase in pressure is obtained by heating the gas in a manner that does not require a pyrotechnic material to generate large quantities of gas, thereby avoiding the use of explosive materials that may be hazardous. Instead, a conventional ignition squib is used to heat the pressurized gas. The squib comprises a high resistance bridging wire and an ignitable or combustible composition that is contained within a suitable container, and is thus separated from the pressurized gas. Upon receipt of an electrical signal, the bridging wire is heated, igniting the composition, which burns within the container and releases heat to the pressurized gas without releasing a large amount of gaseous combustion products. However, a significant amount of the heat generated by the squib is absorbed by the container when the squib is ignited, reducing the thermal efficiency of the inflator means. As a result of the low thermal efficiency, a larger amount of gas must be stored in the inflator means than would otherwise be required if the amount of heat absorbed by the device was minimized, thus raising the thermal efficiency.
U.S. Pat. No. 3,655,217 to Johnson discloses a hybrid inflator as a pressure source for inflatable safety devices. The pressure source includes a generally cylindrical member having an outlet member that is sealed with a rupturable disk welded to one end and a housing welded to the other end to provide a closed chamber for storage of a fluid under pressure, such as air, which is typically stored at a pressure of about 815 psi. A propellant is stored in a counter bore in the housing, and is separated from the pressurized fluid by a retainer cup. Upon receipt of an electrical signal, a conventional squib, mounted in the housing, ignites an ignition composition that, upon combustion, ruptures a closure member between the squib and the propellant, igniting the propellant. The propellant charge then burns, producing hot gas and pressure, which rupture a portion of the retainer cup, allowing the hot gas to flow into the pressurized fluid, heating the fluid, resulting in an increase in pressure of the fluid, bursting the rupturable disk at a pressure of about 1,250 psi, and allowing the pressurized fluid and hot gas to escape from the pressure source. The temperature of an air bag inflated with the disclosed pressure source is typically about 140.degree. F. Because the propellant is stored and burned within a counter bore in the housing, a significant percentage of the released heat will be absorbed by the housing, thereby substantially lowering the thermal efficiency of the device.
U.S. Pat. No. 5,602,361 to Hamilton et al. discloses a hybrid inflator for an automotive inflatable safety system that utilizes both a pressurized gas and a gas generating propellant. The disclosed propellants are gun type propellants that generate large quantities of carbon monoxide and hydrogen gas when burned. These undesirable combustion products are eliminated from the inflation gas by including up to about 20 percent oxygen gas in the pressurized gas, so that the carbon monoxide and hydrogen are converted to carbon dioxide and water by reaction with the oxygen. The inflator includes a gas generator positioned within an inflator housing in which pressurized gas is stored at a pressure of about 3,000 psi. The gas generator contains propellant grains that are separated from the discharge end of the gas generator by a screen and a baffle, which, presumably, are intended to retain particulates within the gas generator during operation. The inflator operates by propelling a projectile through a closure disk to open a passageway between the inflator and an air bag. The projectile then impacts an actuation piston, firing at least one primer, igniting an ignition/booster charge, which, in turn ignites the propellant. As all of the components of the gas generator and the gas generator ignition assembly are contacted by at least one of the burning propellant or the hot combustion products, a large portion of the heat generated by the combustion of the propellant is absorbed by the components rather than being transferred to the inflation gas. As a result, the thermal efficiency of the inflator is low.
In prior art hybrid inflators, a significant amount of heat is absorbed by the mechanical components of the inflator, rather than the inflation gas. Therefore, to compensate for the heat lost to the inflator components, and to provide a given volume and pressure of inflation gas, the amount of inflation gas and/or pyrotechnic material and, thus, the size and weight of the inflator are greater than would be required if the thermal efficiency of the inflator was higher. Additionally, and potentially of greater importance, is the fact that the low efficiency of prior art inflators results in a significant heating of the inflator core and housing. As a result, following the operation of the inflator, the temperature of the inflator is sufficiently high to burn skin on contact, and to reduce the strength of the fabric of the inflatable. This is seriously limits the design of the restraint system. A small cool inflator would allow the system to be configured with the inflatable housed within the inflator, thereby greatly reducing cost, and improving system operations.
Therefore, a need exists for an inflator having a high thermal efficiency, which is, in turn, substantially smaller than prior art inflators, while still being capable of inflating an inflatable belt, side bag, or other inflatable restraint device in a time substantially shorter than that of prior art inflators without significantly increasing the temperature of the mechanical components of the inflator. The present invention is directed to such an inflator, as well as passive restraint systems incorporating the subject inflator.